Chapter 27: Diodes & Diode Applications

12/3/2010

Semiconductors

� Properties are between those of conductors and insulators

� Silicon, germanium, carbon

Types of Semiconductors

� Intrinsic Semiconductor

� Material in its purest form

� Silicon crystal

� Extrinsic Semiconductor

� Semiconductor with other atoms which are called impurity atoms

Doping

� Process of adding impurity atoms

Silicon Properties

� In its pure form silicon has 4 electrons in its outer shell

� Valence electrons

Silicon Atom Structure

Silicon Crystalline Structure

Electron-Hole Pairs

� As silicon is heated some electrons will free themselves and leave vacancies (holes) behind

Electron-Hole Pairs

N-Type Semiconductors

� Silicon doped with a pentavalent atom

� 5 electrons in its outer shell

� Materials include antimony, arsenic, and phosphorus

� Produces an extra electron

� Electrons are the majority carriers

n-Type Semiconductor

p-Type Semiconductor

� Silicon doped with trivalent atoms

� 3 electrons in the valence shell

� Materials include aluminum, boron, and gallium

� Causes holes in the structure

� Absence of electrons

� Holes are the majority carriers

P-Type Semiconductor

Diode

� Created by joining a p- and n- type of semiconductor material

� Allows current to flow in one direction only

Diode

Depletion Zone

� At the p-n junction

� Where the p- and n- semiconductors join

� Electrons migrate to equalize the charge

� Create a depletion zone

� Depleted of all charge carriers

Barrier Potential

� Created by the depletion layer

� Voltage potential between the p and the n layer

� Vb� Germanium - 0.3 volts

� Silicon – 0.7 volts

� Cannot be measured externally

Barrier Potential

Bias

� Control voltage or current

� Forward biasing allows current to flow easily through the diode

� Must be greater than Vb to produce current flow

Forward-Biased Diode

Reverse-Biasing

� Increases the size of the depletion zone

� Pulls electron and holes away from the junction

� Diode does not conduct any current

Reverse-Biased Diode

Leakage Current

� A reverse-biased diode will conduct a small amount of current

� Due to the action of the minority carriers in the p and n layers

� Number of which are determined by the operation temperature

Volt-Ampere Characteristics

Breakdown Voltage, VBR

� Occurs when the diode is reverse-biased

� Voltage at which there is a sudden increase in the leakage current (IR)

� Avalanche effect produced by thermally produced free electrons

� Diodes should not be operated in this voltage range

DC Resistance of a Diode

� RF – DC resistance

� VF – Forward voltage drop

� IF – Forward current

Diode Approximations

� First Approximation

� Second Approximation

� Third Approximation

I. First Approximation

� Treats forward-biased diode as a closed switch with a voltage drop of zero volts.

� Treats a reverse-biased diode as an open switch with zero current flow.

� Ideal Diode Approximation

First Approximation of a Diode

II. Second Approximation

� Treats a forward-biased diode as like an ideal diode with a battery in series

� The battery voltage is 0.7 volts

� Barrier potential (Vb)

� A reverse-biased diode is treated as an open switch

Second Approximation

� No current flow until VF > VB

III. Third Approximation

� Includes bulk resistance (rB)

� Resistance of the p and n material in the diode

Third Approximation

� VF – Forward-biased voltage

� VB – Barrier voltage

� rB – Bulk resistance

Calculating rb

How to Use Approximations

Diode Ratings

� Breakdown Voltage, VBR

� Average Forward Current Rating, I0

� Maximum Forward-Surge Current Rating, IFSM

� Maximum Reverse Current, IR

I. Breakdown Voltage, VBR

� Maximum reverse voltage

� Current avalanche occurs here

� Referred as

� Peak Inverse Voltage (PIV)

� Peak Reverse Voltage (PRV)

� Breakdown Voltage Rating (VBR)

� Peak Reverse Voltage Maximum (VRRM)

II. Average Forward Current

Rating, I0

� Maximum current that a diode can safely handle

III. Maximum Forward-Surge

Current Rating, IFSM

� Maximum instantaneous pulse current that a diode can safely handle

IV. Maximum Reverse Current, IR

� Diode in reverse-bias

� Current at a reverse-bias voltage value

Rectifier Circuits

Function

� Convert alternating current (AC) to direct current (DC)

Half-Wave Rectifier

� Forward-biased on the positive AC peaks

� Reverse-biased on the negative AC peaks

� Frequency of the output voltage is the same as the input voltage

Half-Wave Rectifier Circuit

Half-Wave Rectifier – Reverse Biased Mode

Full-Wave Rectifier

� Uses a center-tapped transformer� Center tap is at ground potential

� Has two diode which are alternately forward and reverse biased� Provides output at both the positive and

negative portions of the AC cycle

� Output voltage has twice the frequency of the input voltage

Full Wave Rectifier

Full-Wave Bridge Rectifier

� Has the same type of output

� Does not require a center-tapped secondary on the transformer

� Output voltage has twice the frequency of the input voltage

Full-Wave Bridge Rectifier

Filtering

� Output from a rectifier is pulsating DC

� Capacitors smooth out the variation in in the output voltage

� Residual variation after filtering is known as ripple voltage

Capacitors in Power Supplies

Calculating the Ripple Voltage

� Vripple – Ripple voltage

� Vout(pk) – Peak output voltage (peak value)

� t – Capacitor discharge time

� C – value of the capacitor

� RL – Load resistor value

Special Diodes

Light-Emitting Diodes (LED)

� Use of doping material can make diodes emit light

� Gallium, phosphorus, arsenic

� Common colors include red, green, yellow, orange, blue, infrared

How LED’s Emit Light

� Diode must forward-biased

� Electrons cross from the n region to the p region fall into holes

� Release energy in the form of light and some heat

� Higher barrier voltage (VB) of 1.5 to 2.5 volts

LED Schematic Symbol

LED Operating Characteristics

� Higher barrier voltage (VB) of 1.5 to 2.5 volts

� Very low voltage breakdown values

� 3 to 15 volts are common

� Can be easily damaged

Typical LED Circuit

LED with Diode Protection

Zener Diode

� Optimized to run in the voltage breakdown region

� At the breakdown voltage, known as the zener voltage, the voltage remains constant as the current increases up to the maximum value

Zener Voltage Operation

Zener Diode Operating Parameters

� Vz – Zener voltage, operating voltage

� IZ – Zener current

� IZT – Zener test current

� IZM – maximum zener current

Zener Diode Formulas

Zener Diode Circuit #1

Zener Diode Circuit #2

Current for a Loaded Zener Circuit